1. Field
Embodiments of the invention relate to the field of semiconductor device fabrication. More particularly, the present invention relates to a hardware plasma interlock system to protect an RF generator from potentially harmful effects that may occur during a plasma failure in an RF generated plasma system.
2. Background
Plasmas are used in a variety of ways in semiconductor processing to implant wafers or substrates with various dopants, to deposit or to etch thin films. Such processes involve the directional deposition or doping of ions on or beneath the surface of a target substrate. Other processes include plasma etching where the directionality of the etching species determines the aspect ratio and the quality of the trenches to be etched.
Plasmas are generated by supplying energy typically from an RF generator device to a neutral gas at low pressure. The ions that are generated by ionization processes in the plasma are implanted into the target substrate. For example, plasma doping (PLAD) systems are typically used when shallow junctions are required in the manufacture of semiconductor devices where lower ion implant energies confine the dopant ions near the surface of the wafer. In these situations, the depth of implantation is related to the bias voltage applied to the wafer. In particular, a wafer is positioned on a platen, which is biased at a negative potential with respect to the grounded plasma chamber. A gas containing the desired dopant materials is introduced into the plasma chamber. Plasma is generated by ionizing the gas atoms and/or molecules.
Once the plasma is generated, a plasma sheath forms between the plasma and the surrounding surfaces, including the workpiece. The sheath is essentially a thin layer at the boundary of the plasma which has a greater density of positive ions (i.e. excess positive charge) as compared to the bulk plasma which is electrically neutral. The platen and substrate (e.g., wafer for doping applications) are then biased with a negative voltage in order to cause the ions from the plasma to cross the plasma sheath. During crossing of the sheath, the ions acquire a kinetic energy equal with the potential drop across the sheath. The ions are implanted into the wafer at a depth proportional to the applied bias voltage. The ion dose implanted into the wafer and the dopant depth profile determines the electrical characteristics of the implanted region and the uniformity of the dose across the wafer surface ensures that all devices on the semiconductor wafer have identical operating characteristics within specified limits. Each of these parameters are critical in the semiconductor fabrication process to ensure that all devices have the desired operating characteristics.
Another approach is to generate plasma in a plasma chamber by coupling RF power from an RF generator to the working gas through an RF antenna. The antenna may be external to the plasma chamber with a dielectric window (alumina, quartz, sapphire, aluminum nitride) that serves as a transferring medium for the electromagnetic radiation generated by the antenna. The working gas is brought into the plasma chamber through gas inlets whereas vacuum pumping is accomplished through an extraction slit that also serves as an ion beam extraction path. The plasma chamber is held at a positive potential and the platen upon which a wafer or target substrate is disposed is at ground potential. In this manner, ions are extracted and the energy which they are implanted into the wafer is given by the potential difference between the plasma source chamber and the ground potential. By adjusting the RF power level and the working gas pressure (e.g. by adjusting the gas flow rate) the extracted ion beam current and its composition can be modified.
During operation of an RF plasma source generator, the plasma in a plasma chamber may be extinguished due to external causes such as, for example, extraction glitch, too low pressure, lack of gas, etc. An extraction glitch may be characterized as a sudden transient in the ion beam current. These glitches may cause unstable ion source operation and ion extraction thereby compromising the desired beam profile that negatively impacts manufacturing throughput. Similarly, too low of a pressure in the RF plasma chamber or a lack of working gas to be ionized may also result in unstable ion source operation and ion beam extraction. Under these circumstances, RF voltage levels on the antenna and RF circuitry may rise leading to undesired electrical arcs followed by matching network and/or RF plasma source generator electronics failure(s) or even fire.
In the absence of plasma, the RF power that would otherwise be transmitted to the gas molecules as ionization or excitation energy, now returns to the RF plasma source generator as reflected RF power. To prevent RF generator failure, fault protection circuitries (e.g., interlocks) that are part of the RF generator electronics are set to shut down the RF power output when reflected RF power exceeds a certain percentage of the forward power (e.g., >20%). The forward power is the power sent by the RF generator through the RF antenna to the load, i.e., plasma.
Various plasma monitoring methods have been proposed to initiate RF plasma source shut-down when abnormal RF operation is observed. Some of these monitoring methods include optical detectors, microwave injection-detection systems, and matching network capacitor tuning utilizing monitoring software algorithms. Unfortunately, these methods are less than ideal since they are either: too complicated, too slow in the response time, or not fully reliable.
For example, in RF driven plasma sources the power from an RF generator is coupled into the plasma by a matching network unit whose role may be to adapt a 50Ω output impedance of the RF generator to the variable plasma impedance level. The most common type of matching network, the so-called L-type configuration, comprises two variable capacitors (10-2000 pF), a tuning capacitor (CT) and a loading capacitor (CL) connected in series and respectively in parallel with the RF antenna in the case of inductively coupled plasma (ICP) or the powered electrode in the case of capacitively coupled plasma (CCP). Three elements Π-type and T-type matching networks are wide band matching networks and make possible matching for almost any type of load.
In a known plasma monitoring method, an electrical detector may be used to sense the amount of reflected RF power. Based on minimization algorithms, two motors drive a variable tuning and a variable loading capacitor to certain values of capacitance designed to achieve a global minimum in the reflected RF power (maximize transmitted power to the load). In other words, the capacitance values of the capacitors in the matching network are adjusted so that the overall impedance of the load and the matching network is 50Ω. Being mechanically driven, the response time of such a feedback system is relatively slow (typically a few seconds) when compared with response times of purely electrical signals.
In another known plasma monitoring method, the plasma state is monitored using an optical detector such as a photodiode. As long as the plasma is present, excited atoms and molecules in the plasma will emit photons and the emitted light can be used as an indicator of plasma presence. Although faster than the mechanical adjustment of capacitors method, the optical detector method is slower than a reflected RF power derivative method because it requires an additional step that involves control system software that adds ˜250 ms to the response time. In addition, the optical detector method is much more complicated. It utilizes additional hardware (e.g., photodiode, windows, electronic filters) and imposes additional limitations (e.g., the necessity of a transparent window, prevention of window coating, limited photodiode lifetime, RF noise picked-up by the photodiode preamplifier, etc.) to the plasma monitoring method.
Another plasma monitoring method is based on microwave detection. Although the microwave detection method solves the window coating problem mentioned above, it requires a microwave generator, an antenna emitter and a microwave detector. Each of these components is difficult to implement on/in a production tool. Moreover, both the microwave and optical methods require a control system software stage to analyze the signal and then send an RF power shut down command to the RF plasma source generator to shut down the output of the RF plasma source generator. As earlier stated, the control system software stage can add ˜250 ms to the response time.